The impact of heating, ventilation and air conditioning (HVAC) design features on the transmission of viruses, including the 2019 novel coronavirus (COVID-19): A systematic review of humidity

The aerosol route has been a pathway for transmission of many viruses. Similarly, recent evidence has determined aerosol transmission for SARS-CoV-2 to be significant. Consequently, public health officials and professionals have sought data regarding the role of Heating, Ventilation, and Air Conditioning (HVAC) features as a means to mitigate transmission of viruses, particularly coronaviruses. Using international standards, a systematic review was conducted to comprehensively identify and synthesize research examining the effect of humidity on transmission of coronaviruses and influenza. The results from 24 relevant studies showed that: increasing from mid (40–60%) to high (>60%) relative humidity (RH) for SARS-CoV-2 was associated with decreased virus survival; although SARS-CoV-2 results appear consistent, coronaviruses do not all behave the same; increasing from low (<40%) to mid RH for influenza was associated with decreased persistence, infectivity, viability, and survival, however effects of increased humidity from mid to high for influenza were not consistent; and medium, temperature, and exposure time were associated with inconsistency in results for both coronaviruses and influenza. Adapting humidity to mitigate virus transmission is complex. When controlling humidity as an HVAC feature, practitioners should take into account virus type and temperature. Future research should also consider the impact of exposure time, temperature, and medium when designing experiments, while also working towards more standardized testing procedures. Clinical trial registration: PROSPERO 2020 CRD42020193968.


Introduction
The World Health Organization (WHO) declared, in March 2020, a global pandemic due to Coronavirus Disease 2019  which is caused by Severe Acute Respiratory Syndrome coronavirus 2 (SARS-CoV-2) [1,2]. Throughout the world, public health authorities  [3], facilitated by virus-laden aerosols, which are expelled by humans, that remain airborne for extended periods of time. Recent evidence suggests that, particularly in indoor environments with poor ventilation, SARS-CoV-2 can spread via airborne transmission [4,5]. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) released a statement in April 2021 declaring that "airborne transmission of SARS-CoV-2 is significant and should be controlled. Changes to building operations, including the operation of heating, ventilating, and air-conditioning systems, can reduce airborne exposures" [6]. As a result, determining the appropriate measures to help protect occupants of indoor spaces based on informed, interdisciplinary research is critical to managing and controlling the spread of infectious disease [7]. Heating, ventilation, and air conditioning (HVAC) systems can be used to mitigate airborne transmission of viruses by diluting or removing the contaminated air where humans breathe from inside the building envelope [7][8][9][10]. HVAC design features, particularly humidity, can influence transmission. As part of their 2021 recommendations for minimizing infectious aerosol exposure, ASH-RAE recommended "maintaining temperatures and humidity at set points," potentially highlighting the role of humidity in transmission [11]. Previous systematic reviews have also noted the impact of humidity on infectious agents [4,12,13]. Derby et al. reviewed the effect of low humidity (�40% relative humidity [RH]) on virus viability and transmission [12] and identified several studies, both modelling and experimental, showing that humidity influenced virus transmission and virus survival [14][15][16][17][18]. Some of these studies found that increasing humidity from low RH levels to approximately 50%RH was associated with decreased transmission [14,15,18]. Other reviews have also highlighted the effect of temperature, exposure time, and air sampling techniques [4,12,13]. While Derby et al. [12] focused on the impact of low humidity levels (<40%RH), they also grouped humidity levels to allow for comparisons across studies: low (20-30%RH), mid (~50%RH), and high (70-90%RH).
Coronaviruses have emerged as infectious agents of great concern for potential airborne transmission. Coronaviruses are lipid enveloped, single-stranded RNA (ssRNA) viruses [19]. Seven human coronaviruses have been identified; however, SARS-CoV-2, Severe Acute Respiratory Syndrome coronavirus (SARS-CoV), and Middle East Respiratory Syndrome coronavirus (MERS-CoV) have received the most attention due to their pathogenicity and lethality [20]. These coronaviruses had their first emergence in the last 18 years [20], with SARS-CoV in 2003, MERS-CoV in 2012, and SARS-CoV-2 in 2019. However, due to the potentially limited number of studies examining coronaviruses, studies examining the influence of humidity on influenza viruses may also provide useful information. As virus envelopes were found to be an important factor in virus transmission [12,13], Influenza (both A and B strains) was chosen for inclusion in the present review due to its structure as a lipid enveloped, ssRNA virus [14].
As mentioned, previous reviews have studied the role of humidity in virus transmission in some capacity [4,12,13]. This systematic review builds on these previous reviews through an extensive and comprehensive search of the literature to identify and synthesize published research determining the impact of humidity in reducing virus transmission. While Derby et al. [12] focused on the role of low humidity, the present review seeks to provide a broader picture including all humidity levels. As well, this review focuses on the enveloped, ssRNA coronaviruses and influenza viruses as opposed to viruses more generally. By doing so, the insight drawn from this review could help answer questions of the role of humidity in SARS-CoV-2 transmission mitigation in mechanically ventilated indoor environments. As well, a detailed examination of the existing scientific literature can identify gaps in current research, which can guide future research priorities.

Methods
As part of a larger research program to review the literature on HVAC design feature and airborne virus transmission, this systematic review was performed to identify and synthesize the scientific literature regarding the impact of humidity on virus transmission within the built environment. Results for other design features of interest (ventilation, ultraviolet radiation, and filtration) are reported separately. The systematic review is registered (CRD42020193968) and a protocol was developed a priori and made publicly available [21,22]. Standards, as defined by the international Cochrane organization [23], for the conduct of systematic reviews were followed with modifications for questions related to etiology [24]. Additionally, the review was reported according to relevant reporting standards [25].

Search strategy
Using concepts related to virus, transmission, and HVAC, a research librarian (GMT) searched three electronic databases (Ovid MEDLINE, Compendex, Web of Science Core) from inception to June 2020 (see Appendix A in S1 File for the Ovid MEDLINE search strategy). Prior to implementing the searches, two librarians peer-reviewed the strategies (TL, AH). An updated search was conducted in January 2021. Reference lists of all relevant papers and review articles were screened. Using Compendex and Web of Science, conference abstracts were identified and were not included, but literature was searched to identify if any relevant abstracts had been published as complete papers. Limits for language or year of publication were not placed on the search. However, only English-language studies were included due to the volume of available literature and resource constraints. References were managed in End-Note and duplicate records removed prior to screening.

Study selection
Study selection occurred in two stages: title/abstract screening and full-text screening. In the first stage, two reviewers independently screened the titles and abstracts of all references identified by the searches of the electronic databases. Relevance of each record was classified as No, Yes, or Maybe. Conflicts between No and Yes/Maybe were resolved by one of the review team. Pilot testing was conducted with three sets of studies (n = 199 each) to develop consistency among the review team. The review team met to discuss discrepancies and develop decision rules after each set of pilot screenings. In the second stage, two reviewers independently reviewed the full-text articles and applied the inclusion/exclusion criteria. Reviewers classified studies as Exclude or Include. Conflicts between Exclude or Include were resolved through consensus by the review team. One reviewer resolved conflicts when different exclusion reasons were given. Pilot tests with three sets of studies (n = 30 each) were used for the second stage of screening. The review team met to resolve discrepancies after each pilot round. Covidence software was used to conduct screening.

Inclusion and exclusion criteria
Exclusion and inclusion criteria are listed in Appendix B in S1 File. This systematic review was part of a larger effort to examine virus transmission and different HVAC design features. While all four design features were included in the search and screening process, only studies evaluating humidity were synthesized here. In addition, literature examining humidity in combination with ultraviolet radiation was addressed in a separate systematic review on ultraviolet radiation. A variety of agents were included in the search with priority placed on studies of viruses or agents that simulated viruses. Other agents (e.g., fungi, bacteria) would be included if studies were not available specific to viruses. Studies using bacteriophages, which are viruses that infect bacterial cells [26], were included. For this specific review, the synthesis was further narrowed from viruses to coronaviruses and influenza viruses. Studies of the indoor built environment (e.g., office, public, residential buildings) which had mechanical ventilation were of particular interest. Primary research providing quantitative results of the association between humidity and virus transmission was included. Only English-language, peer-reviewed publications were included.

Risk of bias assessment
For experimental studies, the risk of bias was determined based on three key domains: selection bias, information bias and confounding [27,28]. Reviewers assessed domains as high, low, or unclear risk of bias using signalling questions [23] for the different study types that were included (e.g., animal studies, laboratory experiments, epidemiological studies) from guidance documents [27][28][29][30]. Modelling studies were assessed using the following three key domains: definition, assumption, and validation [30,31]. Definition considered model complexity and data sources, assumption considered the explanation and description of model assumptions, and validation considered model validation and sensitivity analysis [31]. Reviewers assessed each domain as high, low, or unclear risk of bias based on signalling questions [30][31][32]. Pilot tests were conducted among three review authors for risk of bias items, then two reviewers (DD, EK, or NF) applied the criteria to each relevant study independently and met to resolve discrepancies.

Data extraction
General information about the study (authors, year of publication, country of corresponding author, year of publication, study design) and methods (setting, population [as applicable], intervention set-up, agent studied) was extracted. Details on humidity treatment parameters (where available) were extracted, including relative humidity (RH), absolute humidity (AH), medium, exposure time, and temperature, where applicable. The studies were grouped as aerosolized virus, modelling, animal, and field studies. Quantitative data were extracted, in addition to the results of any tests of statistical significance related to humidity. The primary outcome of interest was quantitative measures of the association between virus transmission and humidity. As such, data on actual transmission were extracted where available (i.e., infections), as well as information regarding virus survival, persistence, infectivity, viral load per hour, concentration, recovery, decay rate, death rate, and virus detection in air. In the animal and aerosolized virus tables, humidity was categorized as low (<40%RH), mid (40-60%RH), and high (>60%RH) RH. Symbols were used to denote high virus survival (+), low virus survival (-), mid virus survival (/) (i.e., between low and high), and no effect ( � ). The abbreviation ND was used when virus was not detected, and NR was used when the virus viability in a particular humidity category was not reported. One reviewer extracted data and a second reviewer verified data for accuracy and completeness using a data extraction form spreadsheet to ensure consistency. The review team discussed discrepancies.

Data synthesis
Meta-analysis was not possible due to heterogeneity across studies in terms of study design, humidity levels tested, outcomes assessed, and results reported. Evidence tables were developed to describe the studies and their results. A narrative synthesis of results was conducted by study grouping (aerosolized virus, modelling, animal, and field studies).

Results
The searches yielded 12,177 unique citations. 2,428 were identified as potentially relevant based on title/abstract screening and 568 met the inclusion criteria (Fig 1). 124 studies were relevant to humidity with 65 relevant to viruses more broadly. Of those 65, 24 were specific to lipid enveloped, ssRNA viruses: coronavirus (n = 6) and influenza (n = 18). Two relevant studies [33,34] were related and are considered as one in the analyses that follow, therefore, 23 studies were synthesized. Studies were published between 1943 and 2020 (median year 2013). The majority of studies (n = 10) were laboratory experiments, with six experimental animal studies, one field observational study, and six modelling studies. Details of individual studies are provided in tables and summarized in the sections that follow; humidity was categorized as low (<40%RH), mid (40-60%RH), and high (>60%RH) RH. Studies were funded by national research funding organizations (n = 15) and public foundations (n = 2), with three studies reporting no external funding and three studies not reporting funding sources.
Two studies that examined SARS-CoV-2 showed different results. Smither et al. [36] found that increased humidity from mid to high RH was associated with increased survival in both artificial saliva (AS) and tissue culture medium (TCM), although survival in TCM was less at higher RH than in AS at the same RH. Schuit et al. [35] found that humidity alone did not significantly affect virus survival. Discrepancy in results could be due to differences in the studies' experimental set-up and test procedures, e.g., exposure time up to 60 [35] vs 90 [36] minutes.
Two studies analyzed the effect humidity on MERS viruses, including MERS-CoV [37] and MERS-CoV isolate HCoV-EMC/2012 [38]. Van Doremalen et al. [37] found increased humidity from mid to high RH was associated with decreased virus survival (i.e., the highest survival was at mid RH), noting a significant effect of humidity on virus survival. Van Doremalen et al. [37] did not test at low RH. Pyankov et al. [38] found that increasing RH was associated with increasing virus survival when coupled with decreasing temperature; these results were statistically significant during the 30-and 60-minute exposure times. Mid RH levels were not tested.
Ijaz et al. [39] examined a full spectrum of RH ranges and found that increasing humidity from low to mid RH was associated with increased virus survival for hCoV-229E, with the highest survival for coronavirus at mid RH. As well, increased humidity from mid to high RH was associated with decreased hCoV-229E survival.
Aerosolized coronaviruses were not consistent as to minimum and maximum survival versus humidity. Two studies found that high humidity was associated with minimum virus survival for MERS-CoV [37] and hCoV-229E at 20±1˚C [39] (70%RH and 80±5%RH, respectively). Two studies found low humidity was associated with minimum survival for MERS-CoV [38] and hCoV-229E at 6±1˚C [39] (24%RH/38˚C and 30±5%RH, respectively). One study found that mid RH was associated with minimum virus survival [36]. Two studies found that mid RH was associated with maximum virus survival (50±5%RH and 40%RH) [37,39] and two studies found that high RH was associated with maximum virus survival (79%RH/ 25˚C and 68-88%RH) [36,38]. Schuit et al. [35] did not find a significant effect of humidity so minimum and maximum survival could not be determined.
For increased humidity from mid to high RH, two studies found decreased survival [18] and significantly decreased infectivity [16]. Harper [33,34] found increased humidity from mid to high RH was associated with decreased viability at 7-8˚C and an exposure time of 23 hours. One study found that increased humidity from mid to high RH was associated with increased survival [40]. Harper [33,34] found increased viability when increasing humidity from mid to high RH at 7-8˚C and an exposure time of 0 and 0.1 hours. Two studies found no significant effect when increasing humidity from mid to high RH [37,41]. As well, Harper [33,34] found similar viability when increasing humidity from mid to high RH at 20.5-24˚C and 32˚C at all exposure times and at 0.5, 1, 4, and 6 hours for 7-8˚C.
Unlike coronaviruses, many of the influenza studies presented consistent results for minimum and maximum survival versus humidity level. Most consistently, four of the six aerosol influenza studies found that low RH was associated with maximum survival (~15%RH and <40%RH, respectively) [18,40], viability (<40%RH) [33,34], and infectivity (23%RH) [16]. For minimum survival, Hemmes et al. [18] found that high RH was associated with minimum survival (~90%RH) and Harper [33,34] found that high RH was associated minimum viability  [33,34] at 20.5-24˚C and 32˚C found that both mid and high RH was associated with minimum virus survival and infectivity as there was little to no difference in survival and infectivity at the two RH ranges (43-73%RH and >40%RH, respectively). Schaffer et al. [40] found that mid RH was associated with maximum survival (40-60%RH). Two studies found that humidity was not associated with any significant difference in infectivity [41] and viability [37], as such, minimum and maximum survival could not be determined.

Modelling studies
Coronaviruses. One modelling study [42] examined the effect of humidity on SARS-CoV-2 in terms of viral survival load per hour to determine optimal temperature/RH pairs for virus inactivation (Table 2). Spena et al. [42] used experimental data from Pyankov et al. [38] and Van Doremalen et al. [37] for MERS-CoV, MERS isolate HCov-EMC, SARS-CoV-1, and SARS-CoV-2 in the development of the model. Spena et al. [42] noted that specific enthalpy is a better predictor of ideal virus control than humidity; their study indicates a target value of 55kJ/kg is optimal. Unfortunately, this target results in high absolute humidity values well above typical set points for mechanical systems. To achieve 55kJ/kg specific enthalpy, HVAC settings require an indoor RH of approximately 93% at 20˚C, decreasing almost linearly to 60%RH at 25˚C (Fig 5 in Spena et al. [42]). Spena et al. [42] indicate a triangular subsector on the psychrometric chart within the ASHRAE recommended quadrangular comfort zone which is both optimal for virus control and comfort. Their findings indicate an important trade-off exists between controlling virus activity and typical building indoor air design parameters.
Three studies found that increased humidity from low to mid RH was associated with decreased virus transmission [43], viability [44], and concentration [17]. Two studies found that increased humidity from mid to high RH was associated with decreased virus transmission [43] and virus concentration [17]. One study found that increased humidity from mid to high RH was associated with increased viability [44]. Koep et al. [46] found that increased AH from 2.67 mb to 9.45 mb AH was associated with decreased virus survival and that increased RH from 40% to 60%RH was associated with decreased survival. Halloran et al. [45] found humidity from 20% to 80%RH was associated with similar transmission probability. Additionally, Halloran et al. [45] found that decreasing temperature from 20˚C to 5˚C was associated with increased transmission probability, while increasing temperature from 20˚C to 30˚C was associated with decreased transmission probability.
Two studies found that increased humidity from mid to high RH was associated with decreased virus persistence at 5˚C, 20˚C, and 27-29˚C [14,47]. However, three other studies found that increased humidity from mid to high RH was associated with increased infectivity at 22.2-23.8˚C [48] and transmission at 20˚C (65%RH) and 23˚C [14,49]. Steel et al. [50] found that increased humidity from low to high RH (mid RH not tested) was associated with decreased transmission at 20˚C and 30˚C.

Field studies
Influenza. One study found no significant effect of absolute and relative humidity on Influenza A and B detection in different settings on a university campus in Hong Kong (Table 4) [51]. Table 5 shows a visual representation of the relative change (" increase, # decrease,-no change) in virus infectivity between low (<40%), mid (40%-60%), and high (>60%) RH.

Risk of bias
All animal and field experimental studies had low risk of bias for the three domains: selection bias, information bias and confounding. Seven of the aerosolized virus experimental studies had low risk of bias for all three domains. For the remaining aerosolized virus experimental studies, one had unclear information bias due to lack of clarity regarding exposure time [18] and one had unclear information bias and high selection bias because the test and tracer material were not identical [33,34]. One was assessed with high risk of bias due to confounding for our comparison of interest because both humidity and temperature were changed, where 79% RH and 25˚C was compared with 24%RH and 38˚C [38]. The six modelling studies had low risk of bias for all three domains: definition, assumption, and validation.

Discussion
This systematic review focussed on the HVAC design feature of humidity and its effect on transmission of coronavirus and influenza, both enveloped, ssRNA viruses. Several important findings were revealed. First, increased humidity from mid to high RH for SARS-CoV-2 was associated with decreased virus survival. Second, although SARS-CoV-2 results appear consistent, coronaviruses do not all behave the same and consistent minimum/maximum survival versus humidity level could not be determined. Third, increased humidity from low to mid RH for influenza was associated with decreased persistence, infectivity, viability, and survival; however, increased humidity from mid to high RH for influenza did not show consistent results. Fourth, low humidity was associated with maximum influenza survival; however, the humidity level for minimum survival was not consistent. Fifth, even though both were enveloped, ssRNA viruses, coronaviruses and influenza did not behave the same. For example, SARS-CoV-2 data found that increased humidity from low to mid RH or mid to high RH was associated with either no effect [35] or increased survival [36], while influenza data, using H1N1 as an example, found that increased humidity from mid to high RH was associated with either no effect [16,37,41] or decreased transmission [50]. Sixth, similar to results reported in previous reviews [12,13] of humidity and viruses, medium, temperature, and exposure time contributed to inconsistency in results for both coronaviruses and influenza. The relationship between airborne virus transmission and relative humidity is decidedly complex. Ijaz et al. [39] propose that airborne survival of vertebrate viruses under various environmental conditions cannot be predicted based on viral structure and composition. According to Lowen et al. [15], there is likely more than one mechanism by which relative humidity affects virus transmission. Not only does relative humidity affect viral particles, it can also have an impact on the host. Lowen et al. [15] suggest that low relative humidity can damage nasal epithelia and reduce mucociliary clearance. This would render the host more susceptible to respiratory virus infections.
Temperature and suspending medium are oftentimes entangled with the effects of relative humidity [33,34,39,50]. However, Hemmes et al. [18] asserted that relative humidity has a larger effect on the survival of aerosolized viruses compared to temperature. Salt and protein concentrations in the suspending medium can have a marked effect on the aerosol stability of a virus [37]. Lester [48] found that decreasing the salt concentration of influenza A virus-lung suspension eliminated the deleterious effect of increasing the relative humidity to 50%. Kormuth et al. [41] found that human bronchial epithelial extracellular material (HBE ECM) protected aerosolized influenza virus from relative humidity dependent decay. They go on to state that protein is most likely protecting the virus from decay but other elements of HBE ECM should not be ruled out.
Relative humidity also affects the settling of virus-containing respiratory droplets. High relative humidity is linked with increased settling [14,17,41], thereby preventing the formation of droplet nuclei [14]. However, Yang and Marr's [17] analysis revealed that relative humidity plays a larger role in virus inactivation than removal through settling.
It is understood that increasing humidification is not feasible in all types of facilities due to existing design limitations [16,46]. Noti et al. [16] suggest that high risk, low humidity areas should be identified during the design and construction phase and appropriate consideration should be given to designs that minimize infection risk. Spena et al. [42] identified a region on the psychrometric chart that satisfies ASHRAE Standard 55's comfort zone requirements while also providing optimal humidity conditions to decrease SARS-CoV-2 survival. In general, Spena et al. [42] suggest increasing humidification of supply air in the winter season and decreasing dehumidification of supply air in the summer season.

Implications for research
Based on the included studies, several key implications for research were found, such as the influence of medium, temperature, and exposure time; the need for statistical analysis to better understand and interpret results; and the need for standardized testing procedures. For example, for SARS-CoV-2, increased humidity from low to high RH was associated with an increase or no change in infectivity, where the difference was attributed to differences in exposure time and/or suspending medium. Interestingly, increased humidity from low to high RH was associated with an increase in infectivity for MERS-CoV, where one study decreased the temperature while increasing the RH. While this is one example, the complexity of other factors can be seen through the review content.
While one study [36] directly compared mediums, it was not the only study to comment on the perceived influence of medium on the results. Smither et al. [36] compared Artificial Saliva (AS) and tissue culture medium (TCM), finding that there were fewer particles observed in AS than in TCM, perhaps contributing to the amount of viable virus present. While the difference in medium has implications for research, practitioners may want to consider the results from AS as they are more applicable to real-world transmission or virus survival scenarios. Additionally, using human bronchial epithelial extracellular material (HBE ECM), Kormuth et al. [41] found no significant effect of humidity on viruses. Kormuth et al. suggested that a lack of results could be due to "protection conferred by supplementation of the viruses with HBE ECM" [41 (p744)]. This theory was further tested using Phi6 both with and without HBE ECM. Additionally, Kormuth et al. questioned "how well the media composition represented that of actual aerosols and droplets expelled by an infected host" [41 (p744)]. As such, researchers should be aware of the influence of medium when testing humidity while also considering their research goals (e.g., choosing a medium such as AS to simulate real-world scenarios).
Five studies used multiple temperatures when examining the influence of humidity on viruses [14,15,33,34,39,50]. The relationship between temperature and humidity is complex [42]. For example, Ijaz et al. found that "the fluidity of the lipid-containing envelope is stabilized at low temperature, thus protecting the virion; however, further studies are needed to explain these phenomena" [39 (p2747)]. As well, the Lowen group [14,15,50] found that humidity and temperature as a combined approach could have an impact on virus survival, suggesting that "influenza virus transmission indoors could potentially be curtailed by simply maintaining room air at warm temperatures (20˚C) and either intermediate (50%) or high (80%) RHs" [14 (p1475)]. Additionally, Harper [33,34] found that high temperatures were associated with the lowest survival at all levels of humidity, finding the influence of humidity "negligible" [34 (p68)]. As a result, researchers should consider the influence of temperature in addition to the influence of humidity.
Exposure time was also found to be a factor when testing the effect of humidity [42]. Schuit et al. [35] questioned whether insufficient exposure time explained the inability to detect a RH relationship for SARS-CoV-2 that was similar to hCoV-229E. Smither et al. [36] found that, in conjunction with the effect of medium, increased exposure time resulted in two patterns of virus survival. For SARS-CoV-2 in AS, increased exposure time was associated with increased differences between virus survival between mid and high RH. In TCM, increased exposure time was associated with decreased differences between survival at mid and high RH. Similarly, Harper [33,34] found that aerosols were able to remain viable for a considerable amount of time "in favourable conditions" [33 (p485)]. However, Harper [33] also noted that favourable conditions vary by virus.
Other research implications arising from this review are the need for reporting of statistical analyses and standard procedures for testing. Only four of the 18 experimental studies included statistical analysis in their presentation of results [16,35,37,41]. Two of the four studies found significant results [16,37]; while two studies found nonsignificant results which the authors attributed to study design issues [35,41]. A limitation of this review is that studies with statistical analyses were presented alongside studies that did not conduct or report statistical analyses in order to allow for a full understanding of the available humidity/virus literature. While attempts were made to ensure clarity when reporting results, differences between survival were reported without supporting statistical analysis from the original documentation and as such, could potentially influence overall findings. Additionally, these findings are further complicated by inconsistencies in testing procedures across the included studies. For example, Schuit et al. [35] attributed their nonsignificant results to short exposure times and Kormuth et al. [41] suggested that medium choice may have affected study outcomes. Additionally, as Derby et al.
[12] previously noted, not all of the studies tested a full spectrum of RH levels. Two experimental studies [36,37] did not test low RH (<40%RH) [36,37] and two experimental studies [38,50] did not test mid RH (40-60%RH). As a result, it can be difficult to make comparisons even among similar viruses.

Implications for practice
In their January 2021 Core Recommendations for Reducing Airborne Infectious Aerosol Exposure ASHRAE recommended "maintaining temperatures and humidity at set points" [11]. Spena et al. [42] considered the ASHRAE comfort zone for domestic and office-like environments in their model of SARS-CoV-2. They found that SARS-CoV-2 infectivity would be effectively suppressed for only a portion of the temperature and RH in the ASHRAE comfort zone. This indicated target zone is of high humidity and would be challenging in buildings where mould/mildew control is important, or in older buildings in cold climates where condensation can be a problem. Cold regions would require very significant humidification efforts when outdoor make-up air has very low absolute humidity at inlet. As this modelling study by Spena et al. [42] was released early in the SARS-CoV-2 pandemic timeline, the model includes data for Phi6, HCoV-EMC (MERS), SARS-CoV, and one SARS-CoV-2 study to provide inputs for SARS-CoV-2 modelling. While the enthalpy was not calculated within the scope of this review, the results of Schuit et al. [35] and Smither et al. [36] potentially do not support the findings of Spena et al. [42]. Schuit et al. [35] did not find a significant effect of humidity, possibly due to short exposure times. However, Smither et al. [36] found that high RH was associated with maximum survival at 19-22˚C, whereas the modelling by Spena et al. [42] suggests that mid to high RH is associated with increased inactivation or decreased survival. As such, it would be interesting to see how the results of Spena et al. [42] change with new research.

Strengths and limitations
Comprehensiveness and use of methods to avoid bias, including an a priori protocol, pre-specification of inclusion/exclusion criteria and involvement of at least two reviewers at all stages, are the strengths of this study. The limitations of this study are the inconsistencies across the included studies regarding statistical analysis and standardized testing procedures. To gain a full understanding of the available literature, included studies with statistical analysis were presented alongside included studies without statistical analysis. Comparison of included studies of the same virus was challenging due to a lack of standardized testing procedures regarding exposure time, temperature, and medium.

Future research and practice priorities
Even though increasing relative humidity from mid to high RH was associated with decreased SARS-CoV-2 survival, all coronaviruses do not behave the same. As a result, blanket prescriptive humidity levels for coronavirus mitigation are difficult to ascertain. While influenza survival varied from mid to high RH, increased humidity from low to mid RH was associated with decreased virus survival with maximum survival at low RH. When controlling humidity as an HVAC feature, practitioners should take into account virus type and temperature. Future research should also consider the impact of exposure time, temperature, and medium when designing experiments, while also working towards more standardized testing procedures and statistical analysis.

Conclusions
This systematic review identified 24 studies examining the role of humidity as an HVAC intervention and its effect on transmission of the lipid enveloped, ssRNA influenza and coronaviruses. Similar to previous reviews [12,13], it was found that while humidity can have an effect on viruses, aerosol medium, temperature, and exposure time can also influence the role of humidity. While SARS-CoV-2 results appear to be consistent as increased humidity from mid to high RH was associated with decreased virus survival, not all coronaviruses behave the same way. Additionally, increasing humidity from low to mid RH for influenza was associated with decreased survival; however, increasing humidity from mid to high RH for influenza was not consistent. When examining humidity as a HVAC intervention, medium, temperature, and exposure time should be considered. As well, due to inconsistencies across viruses, while recommended levels for specific viruses could potentially be determined, generalized approaches to humidity cannot be made.